In the solid state lighting technology, although the light emitting efficiency of the organic solid state lighting, e.g., the organic light-emitting diode (OLED) lighting, is less than that of the inorganic solid state lighting, e.g., the light-emitting diode (LED) lighting, the OLED lighting has some unique properties. For example, the OLED is easy to be manufactured as a surface light source, may be light and thin, may not emit ultraviolet (UV) or infrared (IR), and may not contain mercury. Moreover, the spectra of the OLEDs are closer to that of natural light.
On the other hand, there are many issues related to the organic solid state lighting. For example, the life time of the lighting device should be improved. Moreover, when the organic solid state lighting is used in the illumination application, it needs to operate in the state of high brightness, so that it is urgent to improve the light emitting efficiency of the organic solid state. A tandem device is one of the solutions to improve both the life time and the brightness.